Mully 1 thread showing may or may not be enough. When I first put my rockers on that are like yours I had pushrods that were a little too long and when I put them on and adjusted them to load the lifter properly only about 1 thread was showing and it let the cup of the pushrod hit the rocker body at full lift. Pull your valve covers and turn the engine over to where one is at full lift and look at where the top of the pushrod cup is. If it hits the rocker body that may be your problem. On mine it made a clattering similiar to spark knock.